Electronic information devices is become widely used, and thin and light liquid crystal have been used for mobile phones, PC displays, various apparatuses for industrial applications, on-board displays, handy terminals, or advertisement displays, and the like. Liquid crystal displays in which an input device such as a touch panel or a transparent plate for screen protection is combined with a front surface of a liquid crystal display panel, for example displays of ticket vending machines and automatic teller machines (ATM) and mobile terminals such as mobile phones or tablet PCs, have been widely used.
Most liquid crystal displays include a transmissive liquid crystal display panel, in which RGB (three primary colors of red, green, and blue) color filters are disposed for each pixel, as a display device, and irradiate the liquid crystal display panel with light from a backlight device disposed on the rear surface thereof and controlling a voltage applied to the liquid crystal based on an input signal adjust transmitted light intensity for each pixel, thereby displaying images.
In some of the displays provided with the liquid crystal display panel, a function of adjusting luminance of the backlight device is required depending on brightness of a work environment (such as daytime, nighttime, or office lighting), mode setting for suppressing consumption of a battery, or the like. As means thereof, a method, in which a voltage applied to a light source device is controlled to adjust to a current flowing in the light source device and to raise or lower emission illuminance of the light source, or a pulse width modulation (hereinafter, abbreviated to PWM) method, in which a voltage applied to a light source device is kept at constant, the voltage to the light source device is intermittently turned on/off at a constant frequency, and thereby luminance is adjusted based on a ratio thereof (duty ratio), are generally used.
As a light source which is used as a backlight device, a white light emitting diode (hereinafter, referred to as “LED”) in which a yellow fluorescent substance is combined with a blue LED has been mainly used. In recent years, a remarkable increase in luminance and an increase in emission efficiency of the LED have been realized. Further, a decrease in size, an increase in luminance, and a decrease in power consumption of a liquid crystal display have been achieved by using such white LED. However, when the above-mentioned white LED is used as a light source device, due to wavelength characteristics of emitted light, it is difficult to reproduce a vivid color, specifically red having a specifically long wavelength.
Therefore, a structure is known in which a color reproducible range is broadened by using RGB LEDs having different luminescent colors as light source devices. In such structure, a light emitted from the light sources having different colors is incident on a mixing light guide plate different from a light guide plate, with which the display surface is irradiated, and propagates a sufficient distance, thereby the RGB colors are mixed to omit unevenness and are incident on a light guide plate (Japanese Patent No. 4156919).
On the other hand, an LED device is also widely known in which a color reproducible range is broadened by combining a blue LED with a fluorescent substance having emission characteristics improved at a wavelength in a red region when being excited with the blue LED light source (Japanese Unexamined Patent Application Publication No. 2014-227496).